Glass products, particularly glass containers, have been widely utilized in the fields of foods, medicines, agricultural chemicals and the like. Glass containers have problems that they are fragile due to their lower impact strengths and that the broken pieces of glass cause injury and other accidents due to their very sharp edges.
Recently, methods of coating the surface of glass with a resin layer have been disclosed for the purpose of eliminating the problems described above.
For the purpose of protection of glass, one of such methods (a) comprises coating the surface of glass with a resin layer formed with an aqueous polyurethane composition (Japanese Laid-open Patent Application Showa 52-59657) and another of such methods (b) comprises coating the surface of a glass bottle with an inner coating layer of styrene-butadiene copolymer rubber latex (hereinafter referred to as SBR latex) and an outer coating layer of an aqueous polyurethane formed on the outer surface of the inner coating layer.
The aqueous polyurethane is widely utilized for coating of glass bottles because it has good mechanical strength and weather resistance and hence is useful for repeated utilization of glass bottles. The SBR latex is also widely utilized for coating of glass bottles due to its superior elongation and its good adhesive property to the surface of the glass bottles and is useful for prevention of scattering broken pieces of glass under fracture. Another reason why these materials are widely utilized is that they are favorable in view of the handling and the working environment because they are both aqueous materials.
However, conventional constitutions of the resin laminates for coating glass, as described above, have various problems as follows. Since the aqueous polyurethanes are generally inferior in levelling property, wettability and flow extension property, there are the problems that they forms many uneven parts caused by undulated bands, etc. during the drying process and thus provide the inferior appearance of the bottles prepared. Furthermore, they have inferior resistance to alkalis. When bottles are washed before filling them with drinking water and the like, the dried coating layer is often swollen due to the alkalinity of the washing solution. Thus, the coating layers can not be applied for the repeated use of the glass bottles.
Since the SBR latexes have lower swollen gel strength in the coating and the drying of the surface of the glass bottles, cracks, such as crazes, are easily formed on the surface of the coating to provide the inferior appearance of the glass bottles.
Even when the surface of the SBR coating layer is coated with the above dry coating layer of an aqueous polyurethane, the cracks are seen through the outer layer from outside. It is difficult that the cracks are completely filled with the coating layer and thus the commercial value of the bottles are deteriorated.